1. Field of the Invention
The present invention is related with a navigation apparatus, and more particularly, it is related with a display apparatus of an on-vehicle navigation apparatus.
2. Description of the Related Art
There is a navigation apparatus of so-called self-sustaining type, as a measurement apparatus for various movable bodies, such as a car, an airplane, and a vessel. This self-sustaining type navigation apparatus, is constituted to obtain the present position by obtaining a two dimensional displacement (the amount of the vector) of a movable body, from azimuth data from an azimuth sensor, and speed data from a speed sensor, and by integrating the two dimensional displacement to a standard point. For example, in the case of a car, the navigation apparatus obtains the present position (data), by integrating the integrated travel distance and the integrated azimuth obtained from a travel distance sensor and an azimuth sensor respectively, to the standard point.
More concretely, for example, the navigation apparatus obtains the integrated travel distance by making the number of rotations of a drive shaft correspond to the number of pulses, which are generated by the number-of-rotations sensor attached to the drive shaft, and then by multiplying a distance correction coefficient with the distance computed from the total number of pulses generated from the time at the standard position to the time at the present position. On the other hand, the navigation apparatus obtains the integrated azimuth by integrating the azimuth obtained by the earth magnetism sensor.
There is a GPS (Global Positioning System) navigation apparatus developed as a measurement apparatus using an artificial satellite. This GPS navigation apparatus is constituted to receive electric waves from, generally, three or more GPS satellites, and obtain the pseudo-distance data including a time offset of the receiver between each GPS satellite and the receiving point (self-position), the position data of each GPS satellite, and the present position (data) of the receiving point.
As an actual example of such a measurement apparatus applied to a navigation apparatus, there is a simple apparatus, which numerically indicates the latitude and longitude of the present position, and there is also an advanced apparatus, which displays various data, such as a self-position, a distance to the destination, a velocity etc., on a map picture plane displayed on the picture plane of a CRT (Cathode Ray Tube).
This type of navigation apparatus, which displays various data on the CRT picture plane, is constituted to read out map data including the obtained present position, from a memory media, such as a CD-ROM, to perform map-matching by use of the read out map data and the obtained present position data, to generate picture-plane data, and to display the present self-position, the direction of advance, etc. on the map, which is displayed on the picture plane of the CRT. By this displayed image, a user can grasp the present self-position in association with the map.
Here, the map-matching is explained.
Digitized map data includes information of the azimuth and distance as the inclination and length of a line. The technique of estimating the exact present position by use of this information of azimuth and distance, is called map-matching. Even when the detected azimuth shifts slightly, for example, due to the error of the sensor, if the possibility that the present position is on the road on which the car has been travelling until now is judged to be high, the display without sense of incongruity can be accomplished by correcting the present position onto the road, by virtue of this map-matching technique.
When performing the map-matching in the above mentioned conventional navigation system, all of the roads within the predetermined range, which has a center at the present position, become the object of map-matching.
Therefore, such a road also becomes the object of map-matching, which has little relationship with the road on which the car is presently travelling. For example, such a road, into which the car cannot go no matter how it travels from the present position within the range, or such a road, having a turning point from the road, on which the car is presently travelling, only at a position distant from the present position, becomes the object of map-matching.
For this reason, there are problems that the process time becomes long and the processing speed is lowered to increase the burden on the calculating apparatus, and that the accuracy of map-matching is decreased.